Radio communication systems have existed for over a century. During this period of time, antenna designers have generated a wide variety of antenna designs with the goal of achieving good performance in a variety of operating conditions.
Generally, the goal of the antenna designer when designing, for example, a receiving antenna, is to maximize power transfer between an electromagnetic signal incident on the antenna, and the resulting electrical signal generated by the antenna. The higher the power transfer, the higher the received signal-to-noise ratio, which usually results in better receiver performance.
Also, traditionally, radio receivers have comprised electronic circuitry and a separate receiving antenna interconnected to one another through a suitable cable connection. In such systems, antenna designers must consider the distorting influence of the cable connection and the electronic circuitry on the electromagnetic behavior of the antenna.
More recently, with the advent of small radio systems based on integrated circuit technology, it has become possible to make so-called Radio-Frequency IDentification (RFID) systems, wherein an entire radio receiver is housed in a package much smaller than the receiving antenna. In such systems, the almost-complete elimination of the distorting influence of the cable connection and the electronic circuitry enables novel antenna designs.
So-called passive RFID receivers can be much smaller than the receiving antenna in part because they do not require a power supply. Power to operate the receiver is derived from the received radio signal itself. The signal generated by the receiving antenna is rectified by one or more diodes to yield a direct-current (DC) voltage that is used to power the receiver.
Ideal diodes are perfect conductors when a forward voltage is applied and are perfect insulators when a reverse voltage is applied. Real diodes only approximate this behavior. In particular, real diodes require a minimum forward voltage before becoming good conductors. Accordingly, the signal generated by the receiving antenna, must have a voltage higher than the minimum required by the diodes, before a DC voltage becomes available to power the RFID receiver.
So, in contrast with traditional antenna design, the goal for the design of passive-RFID-receiver antennas is to maximize not the received-signal power, but rather the received-signal voltage.
It is well known in the art that antennas are reciprocal devices, meaning that an antenna that is used as a transmitting antenna can also be used as a receiving antenna, and vice versa. Furthermore, there is a one-to-one correspondence between the behavior of an antenna used as a receiving antenna and the behavior of the same antenna used as a transmitting antenna. This property of antennas is known in the art as “reciprocity.”
An antenna used as a transmitting antenna accepts an electrical signal applied at an input port and produces a transmitted electromagnetic signal that propagates through three-dimensional space. It is well known in the art how to represent such a transmitted electromagnetic signal as a vector in a vector space, for example, as a superposition of spherical harmonics. The behavior of a transmitting antenna at a given frequency can be fully characterized by reporting, for example, the spherical-harmonic components of the transmitted electromagnetic signal that it generates in response to a test electrical signal at that frequency that is applied to the antenna's input port.
Such a characterization can be used to derive, unambiguously, the behavior of the same antenna when it is used as a receiving antenna. In this case, the input port becomes an output port that generates an output electrical signal in response to an incident electromagnetic signal propagating through three-dimensional space. The incident electromagnetic signal can be specified by, for example, by specifying its spherical-harmonic components. The resulting electrical signal can then be derived through a scalar product with the spherical-harmonic components of the transmitted electromagnetic signal at the same frequency, as is well known in the art.
A consequence of reciprocity is that an antenna can be fully characterized in terms of its properties as either a transmitting antenna or as a receiving antenna. A full characterization of an antenna when used in one mode (transmitting or receiving) uniquely and unambiguously defines the properties of the antenna when used in the other mode.
For example, in order to understand or measure the radiation pattern of an antenna it is frequently easier to feed an electric signal into the antenna and then observe the electromagnetic field generated by the antenna. This task can be performed experimentally or computationally. The radiation pattern of the antenna that is obtained through this method also applies when the antenna is used as a receiving antenna. Hereinafter, antennas will be interchangeably referred to as receiving or transmitting, and their properties will be discussed as they apply to either transmission or reception, as convenient to achieve clarity. It will be clear to those skilled in the art how to apply what is said about an antenna used in one mode (receiving or transmitting) to the same antenna used in the other mode.
FIG. 1 depicts monopole antenna 100 in accordance with the prior art. Monopole antenna 100 comprises monopole 110, ground plane 120 and co-axial cable connection 130. Monopole antenna 100 is a very common type of antenna and is representative of how many antennas operate. When an electrical signal is applied to co-axial cable connection 130, an electric field appears between monopole 110 and ground plane 120. If the electrical signal has a frequency at or near the so-called “resonant” frequency of the antenna, a large fraction of the power of the electrical signal is converted into an electromagnetic signal that is radiated by the antenna. If the electrical signal has a frequency that is substantially different from the resonant frequency of the antenna, a relatively small fraction of the signal's power is radiated; most of the power is reflected back into the co-axial cable connection.
In principle, it is possible to make an antenna that radiates efficiently at many frequencies, without exhibiting a band of resonance. In practice, it is difficult to make such antennas, and resonant structures (hereinafter also referred to as “resonators”) are commonly used to make antennas that radiate efficiently.
FIG. 2 depicts resonant structure 200, which is an example of a type of resonant structure commonly used to make antennas in the prior art. Resonant structure 200 comprises a length of wire 240 bent in the shape of the letter U, with an input-output port 220 comprising connection points 230-1 and 230-2. As depicted in FIG. 2, the two connection points are attached to the two ends of the wire.
The frequency of resonance of resonant structure 200 depends on its length. The structure can be modeled as a twin-lead transmission line 210 with a short at one end (i.e., the end opposite input-output port 220). The structure is resonant at a frequency for which the length of the transmission line is about one quarter of a wavelength. The range of frequencies near the resonant frequency over which the resonant structure exhibits acceptably good performance is known as the “band of resonance.”
Resonant structure 200 exhibits resonance in a manner similar to monopole antenna 100. Near the resonant frequency, the electromagnetic fields generated by the voltages and currents on wire 240 become stronger, and a larger fraction of the power of an electrical signal applied to input-output port 220 is radiated as an electromagnetic signal. Accordingly, resonant structures that exhibit this behavior are referred to as “electromagnetically-resonant.”
FIG. 3 depicts folded-dipole antenna 300, which is an example of a common type of antenna in the prior art. Folded-dipole antenna 300 can be modeled as being composed of two instances of resonant structure 200 connected in series. When used as a transmitting antenna, an electrical signal is applied through balanced transmission line 320.
Although folded-dipole antenna 300 can be modeled as being composed of two instances of resonant structure 200 connected in series, the signal that it generates when used as a receiving antenna is not the sum of the signals that each instance of resonant structure 200 would generate if used by itself because of the mutual coupling between the two instances of resonant structure 200.
FIG. 4 depicts antenna-with-load-element 400, which is an example of a type of antenna in the prior art for RFID systems known as RFID tags. Antenna-with-load-element 400 comprises: conductive sheets 410-1, and 410-2, electrical connection 420, connection points 440-1 and 440-2, and load element 430, interrelated as shown.
Conductive sheets 410-1 and 410-2, together with electrical connection 420, form resonant structure 450. Load element 430 receives the signal generated by resonant structure 450 through connection points 440-1 and 440-2. When used to implement an RFID tag, load element 430 is small relatively to the size of conductive sheets 410-1 and 410-2.
To implement an RFID tag, load element 430 acts as both a receiver and a transmitter. In particular, in a passive RFID tag, transmission is accomplished through a technique known as “modulated backscatter” wherein load element 430 controls the impedance that it presents to the received signal. Modulated backscatter is based on the fact that, in any radio receiver, a portion of the electromagnetic signal incident on the receiving antenna is reflected. The amplitude and phase of the reflected signal depend on the impedance connected to the antenna port, so that load element 430 modulates the reflected signal by controlling its own impedance.
The impedance of an RFID load element depends on the design and implementation of the device and, typically, it is non-linear, meaning that is varies as a function of the amplitude of the applied signal. As a consequence, the goal of maximizing received-signal voltage is difficult to achieve. There is a need for methods to couple an antenna to an RFID load element that achieve the desired impedance match.